1. Technical Field
This invention relates generally to fabricating electrical components and more specifically relates to the testing of circuits on a printed circuit board.
2. Background Art
Electronic devices have become a vital part of everyday life in this country and throughout the world. Most electronic devices contain large numbers of digital electronic components and analog circuit components that work together to perform various logical operations. The various individual components in many electronic devices are typically linked into circuits on layered printed circuit boards (PCBs). These commonplace electrical circuits control everything from computers to cars to garage door openers to satellites. As new devices are developed, the capabilities of the resulting electronic circuits and components provide increased functionality, usually at a lower cost.
As with any industry, electronic and electrical products are tested prior to introduction into the marketplace. Typically, most electronic and electrical products are tested at least twice, once during the product design phase (for feasibility) and again during the production phase (for quality control). Obviously, the testing and validation process used to test the electrical circuits and the printed circuit boards is not error-free. Ideally, once a product has been tested for feasibility and is being produced, product testing of the electrical circuits and the PCBs should be very straightforward. Unfortunately, while the theory of product testing is sound, the actual testing of circuit and PCBs is often subject to real world constraints that may hinder accurate and complete testing results.
For instance, the gap between the ideal of complete testing and the reality of partial testing of a circuit or PCB is due, in part, to the fact that the primary goal of circuit and board design is functionality. While it may be desirable to add multiple test points on a PCB to facilitate production testing, it will not always be possible to do so. The inclusion of a traditional test point requires that the PCB contain an electrical contact point on the board that may be accessed by an external test apparatus. Each electrical contact point occupies precious space and may impede board functionality. Thus, testability may be lost in a tradeoff with functionality.
In addition, the product design engineers responsible for designing the circuits and PCBs are typically not the individuals responsible for testing the circuits and PCBs. The design engineers may not place a high emphasis on production testability and, therefore, may not incorporate sufficient test points. The addition of new test points after initial design has been completed may require the design engineers to repeat considerable work, thereby increasing costs and delaying product introduction.
Also, once a circuit or PCB design has been approved by management, design changes to include additional desirable test points may, as a practical matter, no longer be feasible. Finally, once a PCB has been assembled, it is typically coated with a sealant known as "procoat" which covers almost the entire surface of the PCB, leaving only the pre-designated testing contact points exposed. The procoat is designed to protect the PCB and to keep dust and other contaminants from interfering with the functional operations of the PCB. In most typical applications, procoat also electrically isolates any surface traces from exterior surfaces.
These practical realities are somewhat disconcerting to a production test engineer who is given the responsibility to ensure quality control for shipping products. The goal of the test engineer is to test the functionality of every major circuit or component on a given PCB. In order to achieve this goal, the production test engineer may desire to create additional test points on a manufactured PCB to allow for more thorough testing of the various circuits and components on the PCB. However, due to the practical constraints explained above, this is typically very difficult to accomplish, if not impossible.
Therefore, there is a need for improved methods which will allow for increased testability of PCBs at the manufacturing stage without impairing board function or excessively increasing the cost or complexity of the manufacturing process. Without new methods or techniques to allow additional test points to be created on a given PCB, the electronics industry will continue to be limited in the ability to improve the reliability of newly designed and manufactured products.